1. Field of the Invention
This invention relates to a developing apparatus, an apparatus unit and an image forming method by which an electrostatic latent image formed on an electrostatic latent image bearing member used in electrophotography, electrostatic recording or magnetic recording is developed to render it visible by the use of a developer carried and transported on a developer carrying member.
2. Related Background Art
A number of methods are conventionally known as electrophotography. In general, copies are obtained by forming an electrostatic latent image on an electrostatic latent image bearing member (photosensitive member) by utilizing a photoconductive material and by various means, subsequently developing the electrostatic latent image by the use of a developer having a toner, to make it visible to form a toner image, transferring the toner image to a transfer medium such as paper as occasion calls, and then fixing the toner image to the transfer medium by the action of heat, pressure or the like.
Development systems in electrophotography are chiefly grouped into one-component type development and two-component type development. In recent years, electrophotographic apparatus are demanded to be made light-weight and small-sized. Accordingly, since the part of a developing apparatus or assembly must be made small, a developing apparatus employing a one-component type development carried out using one-component type developers are used in many cases.
More specifically, such one-component type development systems require no carrier particles such as glass beads or iron powder required in two-component type development systems, and hence can make developing assemblies themselves small-sized and light-weight. Also, since in the two-component type development systems the concentration of toner in the two-component type developer must be kept constant, a device for detecting toner concentration so as to supply the toner in the desired quantity is required, thus, in the case of the two-component type development systems, this also tends to make developing assemblies have larger size and weight. On the other hand, in the one-component type development systems, such a device is not required, and hence the developing assemblies can also be made small-sized and light-weight.
Known as the one-component type development system making use of one-component type developers is, e.g., a system in which an electrostatic latent image is formed on a photosensitive drum serving as a latent image bearing member, positive or negative electric charges are imparted to a toner serving as a one-component type developer, by the friction between a developing sleeve as a developer carrying member and the toner and/or the friction between a developer layer-thickness regulating member for regulating toner coat quantity on the developing sleeve and the toner, then, by this developing sleeve thin coated with the toner on its surface, the toner standing positively or negatively charged is transported to a developing zone at which the photosensitive drum and the developing sleeve face each other, and in the developing zone the toner is caused to fly and adhere to the electrostatic latent image formed on the surface of the photosensitive drum, to perform development to make the electrostatic latent image visible as a toner image.
The one-component type developers used in such one-component type development system include a one-component type magnetic developer containing a magnetic material for making the one-component type developer carried on the developer carrying member chiefly by the action of magnetic force, and a one-component type non-magnetic developer containing no magnetic material. In the latter case, the one-component type non-magnetic developer is carried on the developer carrying member chiefly by the action of electrostatic force. The one-component type developers also include, from their charge polarities, those having a negatively chargeable toner and those having a positively chargeable toner. Then, when, e.g., development is performed on an OPC (organic photoconductor) photosensitive member holding on its surface a negatively charged electrostatic latent image, a developer having a positively chargeable toner is used when what is called regular development is performed, and a developer having a negatively chargeable toner is used when what is called reverse development is performed. Negatively charging OPC photosensitive members are widely used because they have stable performance and are available at a low price. Thus, in printers and digital copying machines, the developer having a negatively chargeable toner is used in many cases since the reverse development is performed. In analog copying machines which performs regular development, the developer having a positively chargeable toner is used in many cases since the regular development is performed.
As the developer carrying member having the function to carry and transport such a developer on the surface when the development is performed, a member is used which is produced by molding, e.g., a metal, an alloy or compound thereof into a cylinder and treating its surface by electrolysis, blasting or filing so as to have a stated surface roughness. When, however, the developer carrying member made of such a material is used and the developer layer is regulated by the developer layer-thickness regulating member into a thin layer and formed on the developer carrying member surface, the developer present on the developer carrying member surface and in the vicinity thereof comes to have a very high electric charge, so that it is strongly attracted to the developer carrying member surface by the action of mirror force. This makes the toner particles have no opportunity of their friction with the developer carrying member, and hence the developer comes to have no preferable electric charges (a phenomenon called xe2x80x9ccharge-upxe2x80x9d). Under such a condition, no satisfactory development and transfer can be performed, resulting in images with much uneven image density and many black spots around line images. Moreover, the toner attracted to the surface of the developer carrying member by such mirror force may trigger spot-like images, what is called blotches which may occur on and adhere to the developer carrying member, or may cause melt-adhesion of toner.
In recent years, developers (toners) are sought to be fixable at a lower temperature for the purpose of energy saving. In such a case, too, it is desired to form highly minute images. In order to realize the fixing of toner at a low temperature, there is, e.g., a tendency that when toners are produced the Tg (glass transition temperature) of the developer is set a little lower or a low-molecular weight component or a low-melting substance such as wax is added to a binder resin in a little larger quantity. When, however, such a toner is used in image formation, the developer tends to melt-adhere to the surface of the developer carrying member because of temperature rise or physical action of the body of an apparatus, consequently tending to cause a decrease in image density, images with white lines and blotchy images.
Japanese Patent Application Laid-open No. 1-112253 and No. 2-284158 disclose a proposal of using toners having small particle diameters so that image quality can be made higher and images can be made more highly minute. Such toners having small particle diameters, however, have a larger surface area per unit weight, and hence tend to have a larger electric charge on the surface, where the toner may stick or adhere to the surface of the developer carrying member because of the phenomenon of what is called xe2x80x9ccharge-upxe2x80x9d, so that the toner fed afresh onto the developer carrying member can be charged with difficulty. In such a case, the toner tends to have a nonuniform charge quantity. This tends to cause sleeve ghost on images, and the resultant images tend to be formed as nonuniform images such as images with lines and fogged images in solid black images and halftone images.
In order to prevent occurrence of such a toner having excessive electric charges and prevent strong adhesion of toner to the developer carrying member, as disclosed in Japanese Patent Application Laid-open No. 1-277256 and No. 3-36570 a method is proposed in which a developer carrying member is formed of a substrate and a coat layer and a conductive material such as carbon black or graphite powder or a solid lubricant is dispersed in the coat layer.
However, this method may be insufficient if employed alone. For example, in recent years, members brought into contact with the photosensitive member are often used in image forming processes. In such a case, a difficulty as stated below may occur. As members brought into contact with the photosensitive member, there are, e.g., a charging member such as a charging rubber roller, a transfer member such as a transfer sponge roller, and a cleaning member such as a cleaning rubber blade. When these members are used, these members come into contact with the photosensitive member, and hence the toner remaining on the photosensitive member or the toner having adhered to these members is pressed against the photosensitive member to tend to cause filming or melt-adhesion.
As a measure against it, as disclosed in Japanese Patent Application Laid-open No. 9-244398 and No. 9-325616, a method is proposed in which a liquid lubricant as exemplified by silicone oil is used in such a way that it is supported on toner constituent materials. As the toner constituent materials made to have the silicone oil supported thereon, there are a magnetic material, a colorant, a charge control agent and also silica used as an external additive, any of which may be used alone or in plurality as so reported. The toner constituted in such a way can be improved in releasability, and not only is effective for preventing, or making less occur, the filming or melt-adhesion stated above, but also can be improved in transfer performance to enable prevention of the phenomenon of blank areas caused by poor transfer (a phenomenon in which the inner area of a line or character image having been transferred is not transferred and stands blank in white). Thus, this is preferably used. However, such a toner in which a liquid lubricant is supported on toner constituent materials tends to have an excessively high charge quantity, and hence tends to cause the phenomenon of charge-up. Especially in the positively chargeable toner, this tendency is strong because the chargeability of the toner has a great dependence on the charge control agent to be added and the external additive to be externally added.
Japanese Patent Application Laid-open No. 5-232793 discloses a developing apparatus comprising a developer carrying member having as a surface layer a resin coat layer which contains at least resin, graphite and carbon black and is so formed that a charge control agent is present in the surface layer and in the vicinity thereof, in order to control the chargeability to toner. Also, as the charge control agent, exemplified are various charge control agents including quaternary ammonium salts. As the resin used in the coat layer formed on the developer carrying member, exemplified are various resins including phenol resins, polyamide resins and polyurethane resins.
However, stated specifically, this prior art shows an Example in which development is carried out using a negatively chargeable toner on the developer carrying member having a resin coat layer employing a phenol resin as the resin and nigrosine as the charge control agent. It has no disclosure at all as to an instance where positively chargeable toners are used and also has neither disclosure nor suggestion at all as to how, in such an instance, positive triboelectric charges can preferably be imparted to the toner when in what combination the resin and the charge control agent are used.
Meanwhile, for the purpose of imparting a high positive charge to toner, Japanese Patent Application Laid-open No. 7-114270 discloses a charge-providing member for developing electrostatic latent images which have at least at part of the surface a quaternary ammonium salt compound having a specific structure. It discloses that the above compound is used together with optionally a binder resin or molding resin component to form a coat layer. As the binder resin or molding resin component, used are styrene resins, styrene-acrylic copolymer resins, polystyrene resins, epoxy resins and mixed resins of any of these, or any of these having an amino group on the alkyl side chain. In its Examples, a styrene-acrylate copolymer resin is used.
However, the developer carrying member having such a coat layer as a charge-providing member may cause contamination by toner or melt-adhesion of toner during many-sheet running, and is sought to be more improved in running performance of the developer carrying member. In addition, according to studies made by the present inventors, in the case of a developer carrying member on which such a coat layer is formed using the quaternary ammonium salt compound and the styrene-acrylate copolymer resin in combination, the quaternary ammonium salt compound is present only in the state it is merely dispersed in the styrene-acrylate copolymer resin. Thus, as shown in Comparative Examples given later in the part of Examples of the present invention, the charging property of the coat layer thus formed is positive chargeability, and hence the ability to impart positive triboelectric charges to positively chargeable toners is also not sufficient.
Accordingly, an object of the present invention is to provide a developing apparatus, an apparatus unit and an image forming method which, in the development of electrostatic latent images by the use of a positively chargeable toner, can make stable the positive-charge-providing properties to toner, may hardly cause any excessive charging of toner and melt-adhesion or contamination of toner onto the developer carrying member, and may hardly cause the image density decrease, faulty images and faulty toner coat (blotches) which may otherwise consequently be caused.
Another object of the present invention is to provide a developing apparatus, an apparatus unit and an image forming method which can form stable images even in repeated image reproduction, and enables development promising a good environmental stability.
To achieve the above objects, the present invention provides a developing apparatus comprising;
a developer container for holding a developer;
a developer carrying member for carrying a positively chargeable developer held in the developer container and transporting the developer to a developing zone; and
a developer layer-thickness regulating member for regulating the thickness of a positively chargeable developer layer to be formed on the developer carrying member;
wherein;
the developer carrying member has at least a substrate and a resin coat layer formed of a resin composition on the surface of the substrate;
the resin composition containing at least (I) a binder resin, (II) a conductive fine powder, (III) spherical particles having a number-average particle diameter of from 0.3 xcexcm to 30 xcexcm and (IV) a quaternary ammonium salt compound which is positively chargeable to iron powder.
The present invention also provides an apparatus unit detachably mountable on the main assembly of an image forming apparatus; the unit comprising;
a developer container for holding a developer;
a developer carrying member for carrying a positively chargeable developer held in the developer container and transporting the developer to a developing zone; and
a developer layer-thickness regulating member for regulating the thickness of a positively chargeable developer layer to be formed on the developer carrying member;
wherein;
the developer carrying member has at least a substrate and a resin coat layer formed of a resin composition on the surface of the substrate;
the resin composition containing at least (I) a binder resin, (II) a conductive fine powder, (III) spherical particles having a number-average particle diameter of from 0.3 xcexcm to 30 xcexcm and (IV) a quaternary ammonium salt compound which is positively chargeable to iron powder.
The present invention still also provides an image forming method comprising the steps of;
a latent image forming step of forming an electrostatic latent image on a latent image bearing member; and
a developing step of developing the electrostatic latent image by the use of a positively chargeable developer of a developing apparatus;
wherein;
in the developing step, the electrostatic latent image is developed by means of the developing apparatus, which comprises;
a developer container for holding a positively chargeable developer;
a developer carrying member for carrying the positively chargeable developer held in the developer container and transporting the developer to a developing zone, wherein;
the developer carrying member has at least a substrate and a resin coat layer formed of a resin composition on the surface of the substrate; the resin composition containing at least (I) a binder resin, (II) a conductive fine powder, (III) spherical particles having a number-average particle diameter of from 0.3 xcexcm to 30 xcexcm and (IV) a quaternary ammonium salt compound which is positively chargeable to iron powder; and
a developer layer-thickness regulating member for regulating the thickness of a positively chargeable developer layer to be formed on the developer carrying member;
the positively chargeable developer being triboelectrically charged by its friction with the surface of the developer carrying member so that positive triboelectric charges are imparted to the positively chargeable developer, and the electrostatic latent image being developed by the use of the positively chargeable developer to which the positive triboelectric charges have been imparted.